1. Field of the Invention
To improve the overall efficiency of a gas turbine engine, a heat exchanger or recuperator can be used to provide heated air for the turbine intake. The heat exchanger operates to transfer heat from the hot exhaust of the turbine engine to the air being drawn into the turbine. As such, the turbine saves fuel it would otherwise expend raising the temperature of the intake air to the combustion temperature.
The heat of the exhaust is transferred, by ducting the hot exhaust gases past the cooler intake air. Typically, the exhaust and intake ducting share multiple common walls, or other structures, to allow the heat to transfer between the ducts. That is, as the exhaust gases pass through the ducts they heat the common walls, which in turn heat the intake air passing on the other side of the walls. Generally, the greater the surface areas of the common walls, the more heat which will transfer between the exhaust and intake air.
2. Description of the Related Art
As shown in the cross-sectional view of FIG. 1, one example of this type of prior art heat exchanger uses a shell assembly 10 to contain and direct the exhaust gases, and a core assembly 20 placed within the shell assembly to contain and direct the intake air. As can be seen, the core assembly 20 is constructed of a stack of thin plates 22 which alternatively channel the inlet air and the exhaust gases through the core 20. That is, the layers 24 of the core 20 alternate between ducting the inlet air and ducting the exhaust gases. In so doing, the ducting keeps the air and exhaust gases from mixing with one another. Generally, to maximize the total heat transfer surface area of the core 20, many closely spaced plates 22 are used to define a multitude of layers 24. Further, each plate 22 is very thin and made of a material with good heat conducting properties. Keeping the plates 22 thin assists in the heat transfer between the hot exhaust gases and the colder inlet air.
The core 20 is contained in the shell assembly 10. Because the shell assembly 10 needs to support the core and is not a heat transfer medium, the shell 10 is typically made of a much thicker material than that of the core 20. Unfortunately, this greater thickness causes the shell assembly 10 to thermally expand at a much slower rate than the quick responding core 20 with its thin plates 22.
With the core 20 held within the shell assembly 10, the loads created by the differential expansion between the core 20 and shell 10 can cause fatigue failures and creep over time. Fatigue and creep can be especially problematic when heat exchangers are repeatedly cycled between hot and cold stages. Depending on their specific use, such turbines can be started, ran-up and shutdown over and over. One example of such cyclic use, is turbines employed in the production of electric power, which are ran only during recurring periods of peak power demand.
An additional problem is the potential for the exhaust gas to bypass the core, instead of traveling through the core. If allowed, some, if not most, of the exhaust gas will divert around an end or the sides of the core. Even a small gap existing between the core and the shell can allow a great deal of exhaust gas to bypass the core. Of course, when the exhaust gas bypasses the core, the rate of heat transfer is lowered, and as a result, the overall efficiency of the turbine and recuperator system drops dramatically.
Therefore, a need exists for a heat exchanger, which allows for differential thermal expansion between the core and the shell assembly, while at the same time maximizing the heat transfer efficiency of the exchanger by preventing the exhaust gases from bypassing the core.
The present invention is embodied in an apparatus, which allows differential thermal expansion while preventing gas from bypassing the core. In at least one embodiment, the heat exchanger includes a shell for containing a first gas, a core positioned within the shell, and a seal positioned between the core and the shell. The seal allows at least some differential expansion between the shell and the core, while restricting the flow of the first gas past the seal. The seal provides a sealed expansion space to exist between the core and the shell. The seal prevents the first gas from bypassing the core by passing through the expansion space. As such, the seal forces the first gas to pass through the core. This greatly increases the heat transfer from the first gas to the core. Preferably, the seal is mounted to the core at a position at least adjacent to the free (moveable) end of the core and about the expansion space.
In one embodiment, the seal is one or more flexible sheets of material at least partially folded to allow for the differential expansion. The seal includes a first end, a second end and fold(s), positioned between the ends. In one embodiment, one or more folds of the material abut against the shell and/or the core to form a seal. Preferably, the material is layered, being folded over at the ends of the layers. The folds on one side of the seal abut the core and the folds on the opposing side of the seal abut the shell. In this embodiment, when the core expands or contracts relative to the shell, the seal is either partly drawn apart (unfolded) or further compacted, as the case may be. As the seal is drawn apart, sufficient material is kept folded between the core and the shell. This allows an acceptable seal to be maintained, preventing, or at least limiting, the first gas from bypassing the core.
In an another embodiment, the seal is one or more sheets of material, which are connected between the core and the seal, without layering by folding. In this embodiment, sufficient extra seal material is provided between the core and the shell to allow the core to expand and/or contract. That is, the seal has enough slack to allow the extra seal material to be taken up during expansion or contraction, as the case may be. Preferably, the seal of this embodiment uses just a single layer of material to substantially prevent the first gas from passing through the seal.
In an other embodiment of the invention, the heat exchanger includes: a shell for containing a first gas flowing through the shell; an expandable core positioned within the shell, where the core has a contracted length, an expanded length, a fixed end mounted to the shell, and a free end separate from the shell, so that the core may expand to the expanded length without being substantially restricted by the shell; an adjustable seal positioned between the core and the shell, where the seal restricts the flow of the first gas past the seal, where the seal is substantially contacting the core at least adjacent to the free end of the core, and where the seal is sufficiently adjustable to allow the core to expand and contract while restricting the flow of the first gas past the seal.
Although the seal can be used with a vast variety of core and shell configurations, it is preferred that the core is a set of plates which define alternating first and second gas layers. The core ducts the first gas from the shell through the core and back out to the shell. Also, the core ducts the second gas from an intake through the alternating second gas layers and out an outlet. This allows heat to transfer from one gas to the other. Preferably, the first gas is a relatively hot turbine exhaust gas (the turbine being connected at its intake and outlet to the heat exchanger) and the second gas is a relatively cool turbine inlet air.